Ive only just noticed this, but the adverts that pop up on the right of my screen on Mumsnet are related to threads you post on.
I've posted on the printer cartridge thread and just now an advert for cartridges popped up. The same happened on a wedding thread now I think about it, the adverts were about hen dos etc.
I know it's actually clever marketing and it's all through key words etc but it is a bit too 'Big Brother' for my liking.